Bioreactor system for multi-stage biological wastewater treatment

ABSTRACT

A bioreactor system for microbiological treatment of wastewater including: at least one highly-flexible substrate for providing a superficial environment conducive for supporting biomass growth, the substrate including: (a) a large plurality of threads disposed in a generally longitudinal direction and including a plurality of synthetic filaments, and (b) at least two cross-support elements disposed across and associated with the plurality of threads, so as to provide support for and loosely associate the threads, wherein openings defined by (i) adjacent threads of the threads, and (ii) adjacent cross-support elements of the elements, have an aspect ratio exceeding 50 to 1.

FIELD AND BACKGROUND OF THE INVENTION

The present invention relates to biological wastewater treatment systems, and more particularly, to a bioreactor system for multi-stage biological wastewater treatment based on spatial microorganisms successions and trophic hydrobionts chains.

It is known that a spatially segregated trophic microorganism chain provides conditions at which larger organisms consume smaller ones. Such a spatial microorganism succession forms a basis for purification processes by means of both aerobic and anaerobic destruction of microorganisms. The result of such a succession is an increased efficiency of biochemical treatment and a reduced quantity of surplus biomass. Such purification processes are suitable for both domestic and industrial wastewaters, even those containing high levels of organic and inorganic impurities. Characteristically, systems for wastewater treatment by trophic microorganism chains include bioreactors having modular spatial aerobic and anaerobic units. Each bioreactor is provided with a controlled air supply that maintains the oxygen level needed for the activity of the microorganisms and enhances the biomass exchange.

Prior art bioreactors suffer from various drawbacks. Trickling filters require a large space, generate secondary pollution including bad odors, and attract flies. Activated sludge processes generate large amounts of biomass that need careful monitoring due to sudden changes in biomass loading and plugging. Rotational bioreactors are more compact, however, they are expensive and prone to mechanical problems.

It is known that bioreactors using fixed submerged biomass usually perform well at low biomass loadings, but are easily plugged by excessive buildup of biomass, therefore, demanding periodic cleaning or replacing of the submerged biomass. These prior art fixed submerged biomass bioreactors require many bioreactors to keep the loading low to maintain the purification efficiency. Consequently these wastewater-treating facilities demand frequent monitoring, good control of flow and load, and are expensive to install, operate and maintain.

U.S. Pat. No. 4,005,010 to Lunt describes a wastewater treatment system having mesh sacks containing a biological medium. The sacks are apparently designed to hold the microbes while allowing fluids to pass through. The biological medium is prone to clogging over the course of operation.

U.S. Pat. No. 4,165,281 to Kuriyama, et al., describes a wastewater treatment system that includes a substrate designed to contain the microorganisms. A plurality of vertically disposed substrates is designed for wastewater to pass therethrough. The likelihood of plugging is greater in this unit than in the Lunt device, due to the orientation of the substrates and to the difficulty in maintaining and/or replacing them.

U.S. Pat. No. 4,279,753 to Nielson, et al., describes the arrangement of a plurality of treatment reactors alternating from aerobic to anaerobic action. While Nielson indicates that it is necessary to address plugging problems, the technique for doing so is relatively crude and appears to be less than completely effective.

U.S. Pat. No. 4,521,311 to Fuchs, et al., teaches the use of a filtering bed through which the wastewater passes and which includes support bedding to suspend the biological medium. The device has a rather complex recirculation process in order to ensure cleaning of the bedding and the microbes. This device may experience additional kinds of clogging problems, and the disclosed bedding particles are required to go through a costly maintenance operation.

U.S. Pat. No. 5,221,470 to McKinney describes a wastewater treatment plant having a final filter made of a sheet of plastic. The sheet of plastic is wrapped about itself so as to form passageways designed for microbe growth. While this design may increase the surface area and, therefore, the dwell time available for microbial action, it is likely that plugging will occur as the passageway fills with dead microbes over a period of time.

In summary, prior-art bioreactor systems for multi-stage biological wastewater treatment are often plagued by inefficiency over a period of operation. When the wastewater to be treated requires the use of a considerable amount of biological mass, there results a problem of plugging of the mass. As waste solids build up on the surface of the mass, or as microbes ingest the pollutants and die, such solids do not always fall to the bottom of the bioreaction tank. Instead, the solids become trapped at or near the surface of the mass. This plugging or blocking of the mass significantly reduces the pathways by which subsequent pollutants may pass through to underlying active microbes that are located below the surface of the mass. Consequently, the acceleration of pollutant decay caused by microbe ingestion is compromised, and water flow through the mass is reduced and may even be stopped. It is therefore necessary to either build a substantially larger bioreactor unit than would otherwise be required—in order to account for this plugging—or to regularly clean the clogged system.

There is therefore a recognized need for, and it would be highly advantageous to have a bioreactor system for multistage biological wastewater treatment that is robust and efficient, simple to operate, insusceptible to plugging, and inexpensive to install and maintain.

SUMMARY OF THE INVENTION

According to the teachings of the present invention there is provided a multiple stage bioreactor system for microbiological treatment of wastewater, the bioreactor system comprising: at least one highly-flexible substrate for providing a superficial environment conducive for supporting biomass growth, the substrate comprising: (a) a large plurality of threads, the threads disposed in a given generally longitudinal direction and including a plurality of synthetic filaments, and (b) at least two cross-support elements disposed across and associated with the plurality of threads, so as to provide support for and loosely associate the threads, wherein the thread segment aspect ratio, which is a ratio of distances between adjacent cross-support elements relative to side-to-side or lateral distances between adjacent threads, exceeds 50 to 1 and the length extension of the threads, which is a ratio expressed in percent, between the length of a section of substrate between adjacent cross-support elements and the average length of the threads forming that section, is in the range of 80% to 98%.

According to another aspect of the present invention there is provided a multi-stage bioreactor system useful for microbiological treatment of domestic and/or industrial wastewater by biomass. The reactor comprises a plurality of successive zones in which are respectively disposed a plurality of flexible biomass-supporting substrates comprising threads disposed in a given general direction. Spaced apart cross-support elements are disposed in angular relationship with said given direction and associated with said plurality of threads, so as to support the threads by engaging the threads at the cross-supports while providing, within distance intervals between the cross-supports, loose association between laterally adjacent thread segments of said substrates. The thread segment aspect ratio, which is a ratio of distances between adjacent cross-support elements relative to side-to-side or lateral distances between adjacent threads, exceeds 50 to 1. The length extension of the threads, which is a ratio expressed in percent, between the length of a section of substrate between adjacent cross-support elements and the average length of the threads forming that section, is in the range of 80% to 98%. A plurality of diffusing elements, fluidly connected with a source of oxygen-containing gas for treatment of the wastewater, is positioned to generate rising gas currents in the wastewater. They have sufficient gas discharge capacity to produce, with said rising currents, a controlled hydrodynamic circulation within said bioreactor. Such circulation is characterized by a linear velocity range bounded by an upper level that enables biomass to settle on said substrates, and bounded by a lower level that provides oxygen sufficient to meet an oxygen demand in said bioreactor.

According to yet another aspect of the present invention there is provided a multi-stage bioreactor system useflul for microbiological treatment of domestic and/or industrial wastewater by biomass. The system comprises a plurality of successive zones in which are respectively disposed a plurality of flexible laminar biomass-supporting substrates. These substrates comprise threads disposed in a generally longitudinal direction and spaced apart cross-support elements disposed in angular relationship with said longitudinal direction. The cross-supports are associated with the plurality of threads, so as to support the threads by engaging them at the cross-supports while providing, within distance intervals between the cross-supports, loose association between laterally adjacent thread segments of said substrates. In this system, the biomass comprises a spatially segregated chain of trophic microorganisms distributed in succession in said zones on the loosely associated flexible biomass-supporting substrates of the respective zones. The thread segment aspect ratio, which is a ratio of distances between adjacent cross-support elements relative to side-to-side or lateral distances between adjacent threads, exceeds 50 to 1. The length extension of the threads, which is a ratio expressed in percent, between the length of a section of substrate between adjacent cross-support elements and the average length of the threads forming that section, is in the range of 80% to 98%. A plurality of diffusing elements is fluidly connected with a source of oxygen-containing gas for treatment of the wastewater and is positioned to generate rising gas currents in the wastewater. They have sufficient gas discharge capacity to produce, with the rising currents, a controlled hydrodynamic circulation within the bioreactor that is characterized by a linear velocity range bounded by an upper level that enables biomass to settle on said substrates and is bounded by a lower level that provides oxygen sufficient to meet an oxygen demand in said bioreactor. The substrate threads are positioned to cause the rising currents to pass along the threads in said generally longitudinal direction.

According to further features in the described preferred embodiments, the aspect ratio exceeds 200 to 1.

According to still further features in the described preferred embodiments, the aspect ratio exceeds 500 to 1.

According to still further features in the described preferred embodiments, the aspect ratio exceeds 2000 to 1.

According to still further features in the described preferred embodiments, the length extension of the threads in the substrate is in a range of 85% to 95%.

According to still further features in the described preferred embodiments, the length extension of the threads is in a range of 90% to 95%.

According to still further features in the described preferred embodiments, the length extension of the threads is in a range of 92% to 95%.

According to still further features in the described preferred embodiments, the threads comprise at least one synthetic material selected from the group consisting of polyamide, polypropylene, and cross-linked polyester.

According to still further features in the described preferred embodiments, the threads comprise polyamide threads.

According to still further features in the described preferred embodiments, the at least two cross-support elements are woven elements interwoven with the threads.

According to still further features in the described preferred embodiments, the woven elements include at least one synthetic material selected from the group consisting of polyamide, polypropylene, and cross-linked polyester.

According to still further features in the described preferred embodiments, the cross-support elements are positioned so as to effect turbulent flow of the wastewater between said substrates.

According to still further features in the described preferred embodiments, the cross-support elements form an angle, with respect to the threads, in a range of 30° to 90°.

According to still further features in the described preferred embodiments, adjacent cross-supports are at least 20 cm apart.

According to still further features in the described preferred embodiments, the substrate threads are positioned to cause the rising currents to pass along the threads in said generally longitudinal direction.

According to still further features in the described preferred embodiments, the biomass comprises a spatially segregated chain of trophic microorganisms distributed in succession in said zones on the loosely associated flexible biomass-supporting substrates of the respective zones.

According to still further features in the described preferred embodiments, a distance between the diffusing elements and the substrates is between 10 and 30 cm, so as to achieve said controlled hydrodynamic circulation.

According to still further features in the described preferred embodiments, a distance between said substrates and a bottom of said bioreactor is between 50 and 80 cm, so as to achieve said controlled hydrodynamic circulation.

According to still further features in the described preferred embodiments, the bioreactor system comprises at least one bundle of flexible substrates.

According to still further features in the described preferred embodiments, the bioreactor system comprises at least two bundles of flexible substrates.

According to still further features in the described preferred embodiments, a single bundle of the at least one bundle includes 2 to 12 of said substrates.

According to still further features in the described preferred embodiments, the substrates of the at least one bundle are disposed in a substantially parallel manner with respect to each other.

According to further features in the described preferred embodiments, a diffusing element is positioned between two bundles of substrates, each of said bundles having an end substrate disposed proximally to said diffusing element, such that said rising current flows upward between said end substrates to effect said hydrodynamic circulation.

According to further features in the described preferred embodiments, the substrate bundle is disposed in an interval of space between adjacent diffusing elements, and wherein the diffusing elements are disposed substantially opposite to large faces of the substrates.

According to still further features in the described preferred embodiments, from 3 to 8 substrates are juxtaposed between adjacent diffusing elements.

According to still further features in the described preferred embodiments, from 4 to 7 of the substrates are juxtaposed between adjacent elements of the diffusing elements.

According to still further features in the described preferred embodiments, the distance between the diffusing elements and the substrates is between 10 and 30 cm, so as to achieve the controlled hydrodynamic circulation.

According to still further features in the described preferred embodiments, the distance between the substrates and a bottom of the bioreactor is between 50 and 80 cm, so as to achieve the controlled hydrodynamic circulation.

According to still further features in the described preferred embodiments, the diffusing elements are further positioned with respect to the plurality of substrates such that a stream of oxygen-containing liquid flows down, through clearances between adjacent substrates, towards a bottom of the bioreactor, and takes part in said hydrodynamic circulation.

According to still further features in the described preferred embodiments, the threads are disposed in a generally longitudinal direction, and wherein each of the substrates further includes at least two cross-support elements, disposed across and associated with the plurality of threads, so as to provide support for and loosely associate the threads, and wherein the cross-support elements are positioned so as to effect turbulent flow of the wastewater between the substrates.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is herein described, by way of example only, with reference to the accompanying drawings. With specific reference now to the drawings in detail, it is stressed that the particulars shown are by way of example and for purposes of illustrative discussion of the preferred embodiments of the present invention only, and are presented in the cause of providing what is believed to be the most useful and readily understood description of the principles and conceptual aspects of the invention. In this regard, no attempt is made to show structural details of the invention in more detail than is necessary for a fundamental understanding of the invention, the description taken with the drawings making apparent to those skilled in the art how the several forms of the invention may be embodied in practice.

In the drawings:

FIG. 1 shows a multi-stage biological activated bioreactor system for wastewater treatment based on spatial microorganisms successions and trophic hydrobionts chains;

FIG. 2 a is a side view of a substrate for biomass buildup;

FIG. 2 b shows a flow regime of liquid within two adjacent substrates;

FIG. 2 c schematically describes the cross-support elements interwoven with threads of the substrates, and

FIG. 3 illustrates the hydrodynamic circulation within a bioreactor, according to the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention is a bioreactor system for treating wastewater, using fixed submerged substrates for the biomass. The bioreactor system is robust and efficient, simple to operate, and highly insusceptible to plugging.

Before explaining at least one embodiment of the invention in detail, it is to be understood that the invention is not limited in its application to the details of construction and the arrangement of the components set forth in the following description or illustrated in the drawing. The invention is capable of other embodiments or of being practiced or carried out in various ways. Also, it is to be understood that the phraseology and terminology employed herein is for the purpose of description and should not be regarded as limiting.

The principles and operation of the system according to the present invention may be better understood with reference to the drawings and the accompanying description.

Referring now to the drawings, FIG. 1 shows a multi-stage biological activated bioreactor system 100 for wastewater treatment based on spatial microorganisms successions and trophic hydrobionts chains. Multi-stage bioreactor system 100 includes a plurality (six are shown) of associated bioreactors 40.

Air supply header 12, connected to an air supply source (not shown in the drawing) supplies the oxygen required in each bioreactor 40 by secondary air lines 14 and check valves 16. These air flows are controlled according to the oxygen demand in each specific bioreactor 40. Secondary air lines 14 deliver air to manifolds 20, disposed near or at a bottom 18 of each bioreactor 40. Manifolds 20 divide the air supply into yet smaller streams, from which emerge upward-flowing air streams via diffusing elements 22 (shown in FIG. 3).

Generally, systems that employ such diffusing elements are superior in treating ability and treating efficiency to a conventional fixed-bed process. Fixed-bed processes, as disclosed, for example, in Sadao Kojima, Yosui To Haisui, Industrial Water and Waste Water, 14, p. 960, (1972) and Y. Maeda, Journal of Fermentation Technology, 53, p. 875 (1975), suffer from excessive clogging of biomass on the supporting media or substrates and from excessive sloughing of falling biomass. Consequently, prior art systems deal mainly with this problem and do not deal with improving the hydrodynamics of the bioreactors.

In sharp contrast to the prior art, one aspect of the present invention focuses on, and appreciably improves, the hydrodynamics of bioreactor 40. Referring now to FIG. 3, air diffused from diffusing elements 22 rises at a linear velocity so as to produce, in bioreactor 40, an “air-lift” effect in which the liquid is carried upwards by the rising air bubbles.

Hydrodynamic circulation within bioreactor 40 is achieved by positioning each diffusing element 22 with respect to substrates 10 such that a major stream of liquid flows up from diffusing element 22 between two substrates 10 that immediately surround diffusing element 22. This causes a hydrodynamic circulation 25 around the top of substrates 10, and down, through the clearances between substrates 10, and through a middle region of substrate bundle 90, towards bottom 18 of bioreactor 40. This hydrodynamic pattern can be advantageously controlled such that a pre-determined minimum level of oxygen is delivered throughout substrates 10, and such that the linear velocity of liquid flowing up from diffusing element 22 is sufficiently low to avoid turbulence and to allow biomass to settle so as to achieve optimal buildup on substrates 10.

It has been found that in order to achieve the inventive hydrodynamic circulation pattern described hereinabove, the number of substrates 10 between adjacent diffusing elements 22 should be in the range of 2 to 14, preferably in the range of 2 to 12, more preferably in the range of 3 to 8, and most preferably, 5 or 6.

Additionally, as will be readily apparent to one skilled in the art, the circulation is influenced and controlled by the number of diffusing elements 22, the distance between them, the distance between diffusing elements 22 and the lower edge of substrates 10, the number of substrates 10 in bioreactors 40, the distance between adjacent substrates 10, and wastewater and air flows.

Multi-stage treatment system 100 is continuously fed with wastewater, or partially-treated wastewater, via inlet 26, which is disposed in an upper part of a first sidewall 28 of system 100. The effluent from the first bioreactor of bioreactors 40 overflows into an adjacent bioreactor through discharge opening 30, located at an opposite side of the first bioreactor, with respect to inlet 26. Similarly, the effluent of the second bioreactor of bioreactors 40 overflows to the third bioreactor of bioreactors 40 through discharge opening 32. Discharge openings 30 and 32 (as well as the discharge openings of all other bioreactors 40) are preferably disposed with respect to one another so as to minimize short-circuiting phenomena within each bioreactor 40.

The treated water produced is discharged from system 100 via outlet 44, which is disposed in an upper part of a second sidewall 46.

Referring now to FIG. 2 a, FIG. 2 a is a side view of a substrate 10 for biomass buildup. Substrate 10 is a loose, laminar, synthetic filter material for supporting biomass growth. Substrate 10, which normally has a height of 1.5 to 4.5 m, a width of 1 to 2 m, and a thickness of only 2 to 10 mm, includes longitudinal threads 50, having a linear density of 100 to 500 dtex, and preferably 200 to 400 dtex. Each thread 50 is typically made up of about 200 to about 500 filaments that provide an enhanced surface area for biomass growth.

Threads 50 are associated and held together by a small number of cross-support elements 52, typically disposed in transverse fashion with respect to threads 50. Each cross-support element 52 is preferably a strip having a width of 2 to 10 cm, and is advantageously formed of interwoven cross threads 80 as shown in FIG. 2 c.

The distance between cross-support elements 52 is at least 20 cm, and preferably, 20 to 50 cm.

In prior-art mesh substrates, the ratio of the length to the width of the mesh openings is 1:1 to 1:3 and more typically, about 1:1. By sharp contrast, the ratio of the length to the width of the openings formed by threads 50 and strips 52, according to the present invention, is greater than 50:1, preferably greater than 200:1, more preferably, greater than 500:1 and even more preferably, greater than 2000:1. Inventive substrate 10 is thus extremely flexible and has openings that, in the lengthwise direction, are large as compared to those of prior-art substrates. Consequently, the biomass has much less firm support, and biomass build-up—which leads to plugging—tends to significantly decrease.

As used herein in the specification and in the claims section that follows, the term “aspect ratio” refers to a ratio of the distance between adjacent cross-supports 52 of a substrate to the lateral or side-to-side dimension from one adjacent thread 50 to another such thread.

Cross-support elements 52 are advantageously thicker than threads 50, so as to restrict water flow 74 between juxtaposed substrates 10, as shown in FIG. 2 b. Thus, instead of the laminar flow characteristic of prior-art systems, the inventive system is characterized by a turbulent flow regime 70 between substrates 10, which enhances the oxygen supply needed for biomass growth.

The length extension of the vertical threads 50 in substrate 10 has also been found to be of paramount importance in attaining optimal biomass build-up. As used herein in the specification and in the claims section that follows, the term “length extension” refers to a ratio, expressed in percent, between the length of a section of a substrate between adjacent cross-support elements (L in FIG. 2 a), and the average length of the threads forming that section. By way of example, in a substrate in which the threads are associated by cross-support elements at the top end and at the bottom end of the substrate, and in which (1) the substrate length L is 0.45 meters, and (2) the average length of the threads is 0.5 meters, the length extension would be equal to 100*0.45/0.5, or 90%.

The length extension of substrate 10 typically exceeds 80%, preferably lies within a range of 85% to 98%, more preferably, within a range of 90% to 95%, and most preferably, within a range of 92% and 95%. Within these narrow ranges of the length extension, adjacent threads 50 have a suitable flexibility, with respect to one another, so as to enable controlled biomass buildup on substrates 10.

Consequently, sloughing is greatly reduced and sloughing of falling biomass as a result of excessive biomass buildup is avoided, as well as excessive biomass loading of down stream bioreactors due to excessive biomass carryover as sloughing from former bioreactors of the multi-stage system.

Threads 50 and cross threads 80 are preferably made of synthetic materials such as polyamide, polypropylene, cross-linked polyester, or any combination thereof. These water-resistant materials enable long-term fabric durability in the aqueous medium, while their low shrinkage properties allow the initial (i.e., at the time of installation) fabric tension of substrates 10 to be preserved.

Typical purification efficiencies of a multi-stage biological activated wastewater treatment system of the present invention are provided in Table 1.

TABLE 1 Purification Efficiency of A Multi-Stage Biological Activated Waste Water Treatment Biological Oxygen Demand (BOD) 97%-99.5% Chemical Oxygen Demand (COD) 85%-95% Total Suspended Solids (TSS) 95%-97% NH₄ ⁺ 90%-99% Oil & Grease 95%-100% Detergents 80%-95%

Although the invention has been described in conjunction with specific embodiments thereof, it is evident that many alternatives, modifications and variations will be apparent to those skilled in the art. Accordingly, it is intended to embrace all such alternatives, modifications and variations that fall within the spirit and broad scope of the appended claims. All publications, patents and patent applications mentioned in this specification, including U.S. Pat. Nos. 4,005,010, 4,165,281, 4,279,753, 4,521,311, and 5,221,470, are herein incorporated in their entirety by reference into the specification, to the same extent as if each individual publication, patent or patent application was specifically and individually indicated to be incorporated herein by reference. In addition, citation or identification of any reference in this application shall not be construed as an admission that such reference is available as prior art to the present invention. 

1. A multi-stage bioreactor system for microbiological treatment of wastewater, the bioreactor system comprising at least one highly-flexible substrate for providing a superficial environment conducive for supporting biomass growth, which substrate comprises a large plurality of threads, the threads being disposed in a generally longitudinal direction and comprising a plurality of synthetic filaments, and at least two spaced apart cross-support elements disposed across and associated with the plurality of threads, so as to provide support for and loosely associate the threads, wherein (A) the thread segment aspect ratio, which is a ratio of distances between adjacent cross-support elements relative to side-to-side or lateral distances between adjacent threads, exceeds 50 to 1 and (B) the length extension of the threads, which is a ratio expressed in percent, between the length of a section of substrate between adjacent cross-support elements and the average length of the threads forming that section, is in the range of 80% to 98%.
 2. The bioreactor system of claim 1, wherein said length extension is in a range of 85% to 95%.
 3. The bioreactor system of claim 2, wherein said threads comprise polyamide threads.
 4. The bioreactor system of claim 1, wherein said length extension is in a range of 90% to 95%.
 5. The bioreactor system of claim 1, wherein said aspect ratio exceeds 200 to
 1. 6. The bioreactor system of claim 5, wherein said aspect ratio exceeds 2000 to
 1. 7. The bioreactor system of claim 1, wherein said aspect ratio exceeds 500 to
 1. 8. The bioreactor system of claim 1, wherein said threads include at least one synthetic material selected from the group consisting of polyamide, polypropylene, and cross-linked polyester.
 9. The bioreactor system of claim 1, wherein said cross-support elements are positioned so as to effect turbulent flow of the wastewater between said substrates.
 10. The bioreactor system of claim 1, wherein said cross-support elements form an angle, with respect to said threads, in a range of 30° to 90°.
 11. The bioreactor system of claim 1, comprising at least one bundle of flexible substrates.
 12. The bioreactor system of claim 11, wherein a single bundle of said at least one bundle includes 2 to 12 of said substrates.
 13. The bioreactor system of claim 1, wherein said substrates are disposed in a substantially parallel manner with respect to each other.
 14. The bioreactor system of claim 1, wherein said threads comprise polyamide threads.
 15. The bioreactor system of claim 1, comprising at least two bundles of flexible substrates.
 16. The bioreactor system of claim 1, wherein adjacent cross-supports are at least 20 cm apart.
 17. A multi-stage bioreactor system useful for microbiological treatment of domestic and industrial wastewaters by biomass, comprising a plurality of successive portions in which are respectively disposed a plurality of flexible biomass-supporting substrates comprising threads disposed in a generally longitudinal direction and spaced apart cross-support elements disposed in angular relationship with said longitudinal direction and associated with said plurality of threads, so as to support the threads by engaging the threads at the cross-supports while providing, within distance intervals between the cross-supports, loose association between laterally adjacent thread segments of said substrates, wherein (A) the thread segment aspect ratio, which is a ratio of distances between adjacent cross-support elements relative to side-to-side or lateral distances between adjacent threads, exceeds 50 to 1, (B) the length extension of the threads, which is a ratio expressed in percent, between the length of a section of substrate between adjacent cross-support elements and the average length of the threads forming that section, is in the range of 80% to 98%, and (C) a plurality of diffusing elements, fluidly connected with a source of oxygen-containing gas for treatment of the wastewater, positioned to generate rising gas currents in the wastewater and having sufficient gas discharge capacity to produce, with said rising currents, a controlled hydrodynamic circulation within said bioreactor characterized by a linear velocity range bounded by an upper level that enables biomass to settle on said substrates, and bounded by a lower level that provides oxygen sufficient to meet an oxygen demand in said bioreactor.
 18. The bioreactor system of claim 17, wherein said length extension is in a range of 85% to 95%.
 19. The bioreactor system of claim 18, wherein said threads comprise polyamide threads.
 20. The bioreactor system of claim 17, wherein said length extension is in a range of 90% to 95%.
 21. The bioreactor system of claim 17, wherein said length extension is in a range of 92% to 95%.
 22. The bioreactor system of claim 17, wherein said aspect ratio exceeds 200 to
 1. 23. The bioreactor system of claim 17, wherein said aspect ratio exceeds 500 to
 1. 24. The bioreactor system of claim 17, wherein said aspect ratio exceeds 2000 to
 1. 25. The bioreactor system of claim 17, wherein the spacing between adjacent cross-supports is at least 20 cm.
 26. The bioreactor system of claim 17, wherein the substrate threads are positioned to cause the rising currents to pass along the threads in said generally longitudinal direction.
 27. The bioreactor system of claim 17, wherein the biomass comprises a spatially segregated chain of trophic microorganisms distributed in succession in said portions on the loosely associated flexible biomass-supporting substrates of the respective portions.
 28. The bioreactor system of claim 17, wherein said threads comprise at least one synthetic material selected from the group consisting of polyamide, polypropylene, and cross-linked polyester.
 29. The bioreactor system of claim 17, wherein a distance between said diffusing elements and said substrates is between 10 and 30 cm, so as to achieve said controlled hydrodynamic circulation.
 30. The bioreactor system of claim 17, wherein a distance between said substrates and a bottom of said bioreactor is between 50 and 80 cm, so as to achieve said controlled hydrodynamic circulation.
 31. The bioreactor system of claim 17, wherein the diffusing elements are further positioned with respect to the plurality of substrates such that a stream of oxygen-containing liquid flows down, through clearances between adjacent substrates, towards a bottom of the bioreactor, and takes part in said hydrodynamic circulation.
 32. The bioreactor system of claim 31, wherein said threads comprise polyamide threads.
 33. The bioreactor system of claim 17, wherein said cross-support elements are positioned so as to effect turbulent flow of the wastewater between said substrates.
 34. The bioreactor system of claim 17, wherein said cross-support elements form an angle, with respect to said threads, in a range of 30° to 90°.
 35. The bioreactor system of claim 17, comprising at least one bundle of flexible substrates.
 36. The bioreactor system of claim 17, comprising at least two bundles of flexible substrates.
 37. The bioreactor system of claim 36, wherein a diffusing element is positioned between said at least two bundles of substrates, each of said bundles having an end substrate disposed proximally to said diffusing element, such that said rising current flows upward between said end substrates to effect said hydrodynamic circulation.
 38. The bioreactor system of claim 36, wherein a particular diffusing element of said plurality of diffusing elements is positioned between said at least two bundles of substrates, each of said bundles having an end substrate disposed proximally to said particular diffusing element, such that a stream of liquid flows between said end substrates to effect said hydrodynamic circulation, and wherein said threads comprise polyamide threads.
 39. The bioreactor system of claim 17, wherein at least one bundle of substrates is disposed between adjacent elements of the difftising elements, and wherein the diffusing elements are disposed substantially opposite to large faces of the substrates.
 40. The bioreactor system of claim 17, wherein a single bundle of said at least one bundle comprises 2 to 12 of said substrates.
 41. The bioreactor system of claim 17, wherein said substrates are disposed in a substantially parallel manner with respect to each other.
 42. The bioreactor system of claim 17, wherein said threads comprise polyamide threads.
 43. The bioreactor system of claim 17, wherein from 3 to 8 substrates are juxtaposed between adjacent diffusing elements.
 44. A multi-stage bioreactor system useful for microbiological treatment of domestic and/or industrial wastewater by biomass, comprising a plurality of successive portions in which are respectively disposed a plurality of flexible biomass-supporting substrates comprising threads disposed in a generally longitudinal direction and spaced apart cross-support elements disposed in angular relationship with said longitudinal direction and associated with said plurality of threads, so as to support the threads by engaging the threads at the cross-supports while providing, within distance intervals between the cross-supports, loose association between laterally adjacent thread segments of said substrates, wherein (A) the biomass comprises a spatially segregated chain of trophic microorganisms distributed in succession in said portions on the loosely associated flexible biomass-supporting substrates of the respective portions, (B) the thread segment aspect ratio, which is a ratio of distances between adjacent cross-support elements relative to side-to-side or lateral distances between adjacent threads, exceeds 50 to 1, (C) the length extension of the threads, which is a ratio expressed in percent, between the length of a section of substrate between adjacent cross-support elements and the average length of the threads forming that section, is in the range of 80% to 98%, (D) a plurality of diffusing elements, fluidly connected with a source of oxygen- containing gas for treatment of the wastewater, positioned to generate rising gas currents in the wastewater and having sufficient gas discharge capacity to produce, with said rising currents, a controlled hydrodynamic circulation within said bioreactor characterized by a linear velocity range bounded by an upper level that enables biomass to settle on said substrates, and bounded by a lower level that provides oxygen sufficient to meet an oxygen demand in said bioreactor, and (E) the substrate threads are positioned to cause the rising currents to pass along the threads in said generally longitudinal direction.
 45. The bioreactor system of claim 44, wherein said length extension is in a range of 85% to 95%.
 46. The bioreactor system of claim 44, wherein said length extension is in a range of 90% to 95%.
 47. The bioreactor system of claim 44, wherein said length extension is in a range of 92% to 95%.
 48. The bioreactor system of claim 44, wherein said aspect ratio exceeds 200 to
 1. 49. The bioreactor system of claim 44, wherein said aspect ratio exceeds 500 to
 1. 50. The bioreactor system of claim 44, wherein said aspect ratio exceeds 2000 to
 1. 51. The bioreactor system of claim 44, wherein the spacing between adjacent cross-supports is at least 20 cm.
 52. The bioreactor system of claim 44, wherein said threads comprise at least one synthetic material selected from the group consisting of polyamide, polypropylene, and cross-linked polyester.
 53. The bioreactor system of claim 44, wherein said threads comprise polyamide threads.
 54. The bioreactor system of claim 44, wherein said diffusing elements are further positioned with respect to said plurality of substrates such that a stream of oxygen-containing liquid flows down, through clearances between adjacent substrates, towards a bottom of said bioreactor, and takes part in said hydrodynamic circulation.
 55. The bioreactor system of claim 44, wherein said cross-support elements are positioned so as to effect turbulent flow of the wastewater between said substrates.
 56. The bioreactor system of claim 44, wherein said cross-support elements form an angle, with respect to said threads, in a range of 30° to 90°.
 57. The bioreactor system of claim 44, wherein the at least two cross-support elements are woven elements interwoven with the threads.
 58. The bioreactor system of claim 57, wherein the woven elements comprise at least one synthetic material selected from the group consisting of polyamide, polypropylene, and cross-linked polyester.
 59. The bioreactor system of claim 44, wherein the portions of the bioreactor system for conducting the respective stages of the process are separated by walls. 